Alkali-silica reaction (ASR) and delayed ettringite formation (DEF) are two causes of concrete deterioration through concrete expansion and cracking. This paper describes the method used to approximate the damage caused by ASR/DEF by mechanically cracking reinforced concrete columns. The structural capacity of a series of columns with varying crack widths was compared to two columns specially cast and exposed to trigger ASR and/or DEF expansion. All columns were tested under doubly eccentric axial loads and failed in bearing. The ASR/DEF columns had over 1% expansion when tested and had no significant reduction in bearing capacity. The mechanical cracking method approximated the ASR/DEF degradation to structural capacity in significantly less time than it took to trigger ASR/DEF in laboratory specimens.

The research program discussed in this paper included both experimental and computational investigations of structural capacity effects on bridge columns by simulating observed column damage. For the experimental research, scaled models of a column were constructed and fractured using stone-splitting wedges. This method was intended to create the worst-case scenario based on the observed damage in the field: cracks propagating through the core of the columns and effectively cleaving each column into four pieces. The finite-element software ATENA, which models cracking in reinforced concrete, was used for the computational modeling and a parametric study. The computer model was correlated to the experimental results and then used to predict capacities for a variety of deterioration levels. This parametric study was used to determine the critical crack width, which would reduce the capacity of the column to its design load. This predicted critical crack width gives the bridge owner another tool for the evaluation of concrete degradation. This paper focuses on the computational portion of the research. As such, this paper presents a method to mimic existing damage in a finite-element model that the practicing engineer could use in a structural assessment of existing conditions.

An experimental study was performed to determine the accuracy of designing a statically indeterminate deep beam with three openings using the strut-and-tie modeling method in accordance with Appendix A of ACI 318-08. In the present study, four strut-and-tie models (STMs) were independently developed to closely match the flow of forces according to a finite element analysis. Four specimens were fabricated based on the associated STM, and confining reinforcement was provided at each load and support point. In all four specimens, the failure load exceeded the factored nominal design strength, demonstrating the conservatism of the STM method. The mode of failure in each specimen was dependent on the stress concentrations revealed in the elastic analysis and the STM chosen for the design.

Results from two sets of full-scale post-tensioned specimens after four and six years of highly aggressive exposure tests are summarized. The study was funded to assess corrosion resistance of both the current state of the industry and possible future developments in strands, ducts, couplers, and anchorages. Non-destructive monitoring was performed throughout the testing, followed by full autopsies of the specimens. The project also served to highlight a new, smaller test specimen and to investigate how the new specimen was able to better isolate design variables. The main focus was durability of galvanized steel ducts in comparison to plastic ducts. While the galvanized duct certainly showed much worse physical behavior, with large sections of the duct being totally breached by corrosion, the plastic duct jointing techniques failed to completely prevent chloride ingress into the grout and therefore to the tendon. Future testing of post-tensioned systems should be conducted with this new procedure and specimen since it was much more cost effective. The project results showed plastic ducts were not subject to corrosion as a result of chlorides but were subject to minor internal gouging damage caused by threading of the strands. Both coupled and uncoupled ducts contained grouts with elevated chloride content. Regardless, the strands showed little physical damage. Additionally the project served to highlight the importance of grouting procedures and the need for proper implementation of the Post-Tensioning Institute grouting standards. Failure to address workmanship issues provided the largest contribution to corrosion damage. These results were considered in a cost analysis which demonstrated how upfront decisions and small initial cost increases can substantially increase performance life and limit corrosion issues.

This paper summarizes a 16-year comprehensive research program carried out at The University of Texas at Austin to identify durability concerns with post-tensioning systems in bridges and to develop durability design guidelines. Major experimental programs examined high performance grouts, segmental joint macrocells, long-term large beam corrosion and long-term column corrosion. After performing comprehensive internal examinations, overall findings indicate unfavorable durability effects due to the use of mixed reinforcement, thin concrete cover, galvanized steel ducts, grout voids and galvanized duct splices. The width of cracks was shown to have a direct adverse effect on specimen performance, and local areas of severe corrosion were found on epoxy-coated and galvanized strands. Favorable behavior was observed, however, for specimens constructed with high-performance concrete, high post-tensioning levels, plastic ducts, and sound, epoxy-filled joints.

In order to complement a large-scale long-term investigation of prestressing strand types for post-tensionsing of bridges, passive and active electrochemical tests were carried out on bare and grouted specimens exposed to salt water conditions as a function of time. The materials tested included seven-wire strands made of hot dip galvanized or zinc coated steel, stainless steel, stainless clad steel, copper clad steel, flow-filled epoxy coated steel and conventional steel as control. Based on corrosion potentials, polarization resistance tests, potentiodynamic tests, weight loss, and visual observations, epoxy coated strands, stainless and stainless clad strands were identified as possible alternatives to conventional steel that might help to minimize corrosion.

The “Building Code Requirements for Structural Concrete” (“Code”) covers the materials, design, and construction of structural concrete used in buildings and where applicable in nonbuilding structures. The Code also covers the strength evaluation of existing concrete structures. Among the subjects covered are: contract documents; inspection; materials; durability requirements; concrete quality, mixing, and placing; formwork; embedded pipes; construction joints; reinforcement details; analysis and design; strength and serviceability; flexural and axial loads; shear and torsion; development and splices of reinforcement; slab systems; walls; footings; precast concrete; composite flexural members; prestressed concrete; shells and folded plate members; strength evaluation of existing structures; provisions for seismic design; structural plain concrete; strut-and-tie modeling in Appendix A; alternative design provisions in Appendix B; alternative load and strength reduction factors in Appendix C; and anchoring to concrete in Appendix D. The quality and testing of materials used in construction are covered by reference to the appropriate ASTM standard specifications. Welding of reinforcement is covered by reference to the appropriate American Welding Society (AWS) standard. Uses of the Code include adoption by reference in general building codes, and earlier editions have been widely used in this manner. The Code is written in a format that allows such reference without change to its language. Therefore, background details or suggestions for carrying out the requirements or intent of the Code portion cannot be included. The Commentary is provided for this purpose. Some of the considerations of the committee in developing the Code portion are discussed within the Commentary, with emphasis given to the explanation of new or revised provisions. Much of the research data referenced in preparing the Code is cited for the user desiring to study individual questions in greater detail. Other documents that provide suggestions for carrying out the requirements of the Code are also cited.

This report summarizes the exploration of new methods of protection for the strands used in post-tensioning of concrete bridges. Companion small-scale tests of strand or grout and strand were conducted. The tests included mechanical testing to determine modulus of elasticity, yield strength, and ultimate strength. In addition a number of passive corrosion exposure tests and accelerated active corrosion tests were run. These accelerated tests included both linear polarization resistance tests and potentiodynamic tests. Strand types investigated included conventional steel strand, hot dip galvanized strand, stainless steel strand, copper-clad strands, stainless-clad strands, and flow-filled epoxy-coated strand. The results indicated that the epoxy-coated strand dominated the others in corrosion resistance and met all mechanical property requirements. The stainless-clad and stainless steel strand were only slightly behind the epoxy-coated in corrosion resistance but need improvement in mechanical properties. The stainless-clad strand met the mechanical requirements for Grade 250 but not Grade 270.

(2009). Eminent Structural Engineer: Dr. Bruno Thurlimann (1923–2008) Structural Engineering International: Vol. 19, No. 1, pp. 102-104.

Emulsifiable oils are often used in posttensioned construction to reduce friction losses and provide temporary corrosion protection for tendons prior to grouting. This paper addresses the effects of two emulsifiable oils and three duct types on bond and friction losses. Bond test results indicate that corrugated galvanized steel ducts provide better anchorage than corrugated HDPE ducts. Rigid steel pipes performed poorly because of failure at the duct-concrete or grout-duct interface. Bond test results also indicate that the ultimate strength of posttensioned specimens with oiled tendons is similar to or better than the ultimate strength of specimens with unoiled tendons. However, specimens with oiled tendons experienced greater slip at a given load than specimens with unoiled tendons. Friction test results indicate that current recommended design values for the coefficient of friction for steel pipes and galvanized ducts are accurate. However, the measured coefficient for HDPE ducts is significantly less than the AASHTO-recommended value. Friction tests also indicate that lubrication of the tendon reduces the friction coefficient by 15% in rigid steel pipes and HDPE ducts if stressing occurs while the oil is fresh.

An experimental study was initiated to observe the strength of reinforced concrete deep beams with openings. Six beams were designed using three strut-and-tie models (STMs) developed according to ACI 318-05. For the first three beams, only the principal reinforcement required for equilibrium was placed in the specimens in general. For the final three beams, minimum distributed reinforcement was placed throughout the specimen, additional anchorage lengths were provided for critical ties, and confining spirals were added to highly-stressed nodal regions. The failure loads of the first three beams ranged from 75 to 84% of the nominal strength requirement. The failure loads of the final three beams ranged from 112 to 160% of the nominal strength requirement. Based on the test results, it was evident that the use of unreinforced bottle-shaped struts and the improper detailing of critical regions of the STM contributed to the unconservative failure loads.

Corrosion protection for bonded, internal, posttensioning tendons (PTTs) can be effective. Within structural elements, PTTs can be well protected by a multilayer system that includes sound design of surface treatments; high-quality concrete; corrosion-resistant ducts; high-quality cementitious grout, coatings, and other internal barriers in the prestressing steel; and superior anchorage protection measures. Potential weaknesses exist in the protection system, however, when the concrete has high permeability or when the concrete is cracked. Additional weaknesses can occur when the posttensioning ducts are not adequately spliced and lack adequate impermeable concrete cover or when the portland cement grout contains voids, bleed water, or cracks. Lastly, the prestressing steel may be more susceptible to corrosion if not adequately protected or handled during construction. Over the past 2 decades, numerous countries reported damage and failure of individual tendons in transportation structures. A thorough research program, initiated at the Phil M. Ferguson Structural Engineering Laboratory, undertook 4 experimental research programs: 1) a study of high performance grouts with the goal of improving the corrosion protection of prestressing strands; 2) a long-term macrocell corrosion test series, initiated to investigate corrosion protection for PTTS to precast concrete segmental construction in which half of the macrocell lab specimens underwent internal examinations after 4-1/2 years and the other half after 8 years of exposure testing; 3) long-term beam corrosion tests that examined the effects of posttensioning and crack width on corrosion protection; and 4) a long-term column corrosion test series initiated to examine corrosion protection in vertical elements. This paper documents the results from the macrocell, beam, and column corrosion test series and recommends durability-design guidelines based on the overall research results.

Strut-and-tie models (STM) are a valuable tool for the design of irregular concrete members. This paper presents the experimental results of tests conducted on small-scale, simply supported dapped beams with openings under the load. The design of each test specimen was developed by independent student teams using the ACI 318-05 provisions for STM. An unreinforced specimen way also constructed for comparison. Each reinforced specimen resisted loads greater than the factored design load and exhibited little distress at service load levels. Examination of the behavior of each model permits appraisal of the design model used by each design team. These results show the STM to be a conservative, lower-bound design approach that allows the designer a great deal of flexibility.

According to a survey conducted in 1996, respondents in several state departments of transportation, indicated that more than 100,000 bridge decks. in the U.S. have-suffered from early age transverse cracking, a crack pattern that typically arises due. to drying shrinkage. (concrete material properties are treated as a means through which to improve the resistance restrained drying shrinkage cracking. Various test methods are discussed as they relate to determining. the resistance of a material to shrinkage cracking. Materials-based methods of controlling drying shrinkage are presented. The materials discussed include fibers, shrinkage-compensating concrete, shrinkage-reducing admixtures, and extensible concrete. It was determined in small laboratory specimens, and confirmed in large-scale bridge deck specimens, that several of the alternative mixtures adequately reduced restrained drying-shrinkage cracking.